1. Field of the Invention
The present invention relates to a printer with a duplex printing capability (hereinafter simply referred to as the “duplex printer”), and more specifically to a stencil printer capable of printing images on both sides of a sheet in a single step.
2. Description of the Background Art
A digital thermal printing system using a stencil is used as a simple, convenient printer. A stencil printer used in such a digital thermal printing system includes a thermal head provided with an array of minute heat-generating elements. While a master is being conveyed in contact with the thermal head, a current is selectively fed to the heat-generating elements in the form of pulses in accordance with image data, thereby perforating the master with heat. After the perforated stencil or master has been wrapped around a porous, cylindrical print drum, a press roller or similar pressing means is pressed against the print drum via a sheet. As a result, ink is transferred from the print drum to the sheet via the perforations of the master, printing an image on the sheet.
Today, duplex printing systems for printing images on both sides of a sheet are becoming widely adopted for such a stencil printer because they reduce the consumption of sheets, the space for storing of documents and so forth. With conventional stencil printers employing such duplex printing systems, it has been customary to execute duplex printing by passing a sheet fed from a sheet feeding section through a printing section to thereby print an image on one side of the sheet, reversing the sheet, and again passing the sheet through the printing section to thereby print another image on the other side of the sheet. However, it is troublesome to again set the sheet driven out at the sheet feeding section or to arrange consecutive sheets each carrying an image on one side thereof by hand. In addition, because sheets are fed to the printing section twice, duplex printing takes twice as much net time as simplex printing, making it too time-consuming.
In light of the above, for example, JP 6-71996 A (pages 3 to 5, FIG. 2) and JP 6-135111 A (pages 4 to 7, FIG. 1) each disclose a stencil printer including first and second print drums and moving means for selectively causing the two print drums into or out of contact with each other. The stencil printer produces a duplex print in a single step by causing the two print drums to contact with each other with the moving means.
Also, for example, JP 8-90893 A (pages 6 to 9, FIG. 1) and JP 8-142477 A (pages 4 and 5, FIG. 4) each propose a stencil printer including a first print drum, first pressing means facing the first print drum via a sheet path, a second print drum positioned downstream of the first print drum in a direction of sheet conveyance and facing the first print drum via a sheet path, and second pressing means facing the second print drum via a sheet path. To produce a duplex print in a single step, the first print drum and first pressing means and the second print drum and second pressing means are sequentially caused to contact each other in this order.
Further, for example, JP 8-332768 A (pages 14 to 20, FIG. 1) teaches a stencil printing method and a stencil printer for practicing the same. The stencil printing method taught in this document produces a duplex print in a single step, by using a master having first and second perforated images formed thereon side by side in the direction of rotation of a print drum. First, a press roller is rotated in direct contact with the print drum in synchronism with either one of the first and second perforated images, so that a first print image corresponding to the first or the second perforated image is transferred from the print drum to the outer peripheral surface of the press roller. Then, the press roller is rotated in contact with the print drum via a sheet with the other of the first and second perforated images being matched in position to the first print image present on the press roller. As a result, the first print image on the press roller is transferred to one side of the sheet while a second print image corresponding to the second image is transferred from the print drum to the other side of the sheet.
The prior art techniques disclosed in JP 6-71996 A, JP 6-135111 A, and the like stated above have the following problems left unsolved. That is, two print drums positioned one above the other are configured to be selectively brought into or out of contact with each other, even in a simplex print mode. In the simplex mode, a perforated master and a non-perforated master must be respectively wrapped around the two print drums, resulting in the wasteful consumption of the master. Further, the two print drums, which are selectively brought into or out of contact with each other each, each have a clamper mounted thereon for retaining the master. Thus, at a position where the clampers face each other, the print drums must be released from each other. This brings about a problem that when printing speed is high, the area over which the print drums contact each other, and therefore an image area, decreases. If the outer diameter of each print drum is increased to guarantee a sufficient image area, then not only the size reduction of the printer is obstructed, but also loud noise is produced when the print drums are brought into contact with each other.
Further, JP 8-90893 A and JP 8-142477 A mentioned above and the like also have the problem that a non-perforated master must be wrapped around one of the two print drums in a simplex print mode, resulting in the wasteful consumption of the master. Another problem is that because the two print drums are serially arranged, the printer is almost twice as large in size as a stencil printer for simplex printing. This is undesirable from the space saving standpoint.
Further, the problem with JP 8-332768 A mentioned above is that image density differs between the front side and the rear side of a sheet because one of the first and second perforated images is directly transferred from the print drum to a sheet while the other of the perforated images is transferred to the sheet by way of the press roller.
Technologies relating to the present invention are also disclosed in, e.g., JP 2003-200645 A.